Looked up using “relajar” vs “relajarse” on Google Translate. Most of it made sense:
I relax = me relajo; we are relaxing = estamos relajándonos; etc. Until I got to this:
the rules are relaxing = las regalas son relajantes:
“relajantes”? Couldn’t find this anywhere on spanishdict for relajar or relajarse. But the muscles are relaxing = los músculos se relajan (makes sense). Only non-reflexive I could come up with was: I relax the muscle = relajo el musculo (but couldn't it be your own?) :)
Yeah that's a bit of a weird one for me; I think the result is wrong
relajante is "relaxing" as an adjective, as in el baño relajante "the relaxing bath"... it means "relaxing" in the sense of "it relaxes (someone)"
It should be las reglas se van relajando "the rules are being relaxed", or something like relajar las reglas "to relax the rules"
I think that's a mistranslation
In general though relajar is "to relax", often "to relax someone else" or "to ease"
And relajarse is "for someone to relax" [or in passive "for something to be relaxed", like relajarse las reglas "for the rules to become relaxed"]
relajante means "relaxing" in the sense of "sedating" or "calming" like a calming effect
As far as "I relax the muscle", it would generally be me relajo el músculo "I relax my muscle" more literally "I relax myself the muscle"
The reflexive is when subject and object are the same; generally your body parts are already assumed to be yours so the reflexive clears that up. It's similar to me lavo las manos "I wash my hands" which is literally "I wash myself the hands"
You could potentially say relajo el músculo (a alguien) "I relax (someone else's) muscle" but that would probably be more medical or chiropractic and not commonplace
In general the objects show "ownership" of the body part, even in indirect objects me duele la garganta "my throat hurts" is actually "it hurts (me) the throat"
I think it's possible to say me relaja el músculo "it relaxes my muscle", or more commonly plural like los termales me relajan los músculos "the hot springs relax my muscles" or el agua caliente me relaja "the hot water relaxes me" but this is an indirect object rather than a reflexive
A reflexive again is subject and object being the same like me relajo "I relax" or me relajo los músculos "I relax my muscles"